Mexico City, a large megacity with over 21 million inhabitants, is
exposed to several hazards, including land subsidence, earthquakes, and
flooding. Hazard assessments for each hazard type is typically treated
separately and usually do not include considerations for any relations among the hazards. Our data makes it plausible for an earthquake triggering case that temporarily accelerated the subsidence rate in the metropolitan area as a result of the Mw 8.2 Tehuantepec and the Mw 7.1 Puebla, September 2017
earthquakes that affected Mexico City. Furthermore, the triggering effect
induced rapid slip along previously developed shallow faults associated with
subsidence. These results indicate that any future scenario of land
subsidence should consider a potential triggering effect by large
earthquakes. Similarly, earthquake hazard assessments should also consider
potential impact on shallow faulting and fracturing associated with land
subsidence.
Mexico City is one of the fastest-subsiding metropolises in the world, where
subsidence rates exceed 360 mm yr-1 (Fig. 1). The subsidence occurs mainly
as the response to aggressive groundwater extraction over the past century,
causing progressive damage to the city's buildings and critical
infrastructure. The subsidence process has been documented for almost a
century (Gayol, 1925; Carrillo, 1948; Figueroa-Vega, 1984; Ortega et al.,
1993). Subsidence rates vary spatially, mainly due to highly heterogeneous
shallow stratigraphic sequence beneath the city (Santoyo-Villa et al., 2005;
Auvinet et al., 2017), which responds differentially to groundwater
extraction. Highest subsidence rates occur above thick layers of lacustrine
sediments (Cabral-Cano et al., 2008; Solano-Rojas et al., 2015), whereas
non-subsiding areas correspond to volcanic rocks. Subsidence occurs
differentially, particularly between stable volcanic rocks and highly
subsiding sediments, producing significant topographic elevation changes and
causing shallow faulting and fracturing along well-defined areas of the city
(Fig. 2).
Sentinel-1 based wrapped differential interferograms for the
Mexico City Metropolitan Area covering (a) 19 July–12 August 2017, (b) 12 August–5 September 2017, and (c) 5–29 September 2017. Color
lookup table for all interferograms depicts 1 cycle = 6.6 cm.
Mexico City subsidence has been studied in the past with conventional
topographic methods (e.g., Auvinet et al., 2017). However, in the past
decade subsidence has monitored by advanced satellite geodetic techniques,
mainly satellite Interferometric Synthetic Aperture Radar (InSAR) and GPS
(e.g. Cabral et al., 2008; López-Quiroz et al., 2009; Osmanoğlu et al., 2011; Du et al., 2019). These studies have routinely detected the highest
subsidence rates in the eastern sector of the city within two areas that
correspond to the former Texcoco and Chalco-Xochimilco lakes (Fig. 1).
Moreover, most of the previous studies focused on the city-wide subsidence
signal, which is important for understanding the city's aquifer mechanics in
response to groundwater extraction. Both GPS and InSAR measurements
indicated limited temporal changes in the subsidence rate throughout the
past two decades with limited seasonal variability.
September 2017 earthquakes
Mexico City recently experienced two large earthquakes that took place only
11 d apart: the Mw 8.2 8 September 2017 with its epicenter offshore Chiapas, and the Mw 7.1 19 September 2017 with its epicenter in Puebla
(Singh et al., 2018). The epicenter of the Mw 8.2 earthquake was located over 700 km
away from Mexico City causing only minor impact to the city. However, the
epicenter of the Mw 7.1 earthquake occurred only ∼ 100 km away
from the city, which resulted in severe damaged in the city. Mexico City is
very susceptible to seismic-induced damage, because part of the city is
built over a sedimentary basin with clay rich lacustrine sediments up to 400 m thick (Santoyo-Villa et al., 2006; Auvinet et al., 2017). This sedimentary
unit amplifies seismic energy causing site effects and making buildings and
infrastructures more vulnerable, depending on their locations within the
city.
Local seismic acceleration and site effects have been previously studied as
part of the city building code developed after the Mw 8.2 1985 earthquake.
However, not much attention has been paid to the shallow faulting associated
with land subsidence that has developed on the transitional zones between
the lacustrine deposits and the volcanic structures outcropping and its role
as a potential risk during large earthquakes. This situation creates a
strong horizontal gradient of subsidence where faulting and fracturing of
the surface is common and is and addition element when considering the
vulnerability for civil structural damage.
Remotely triggered subsidence acceleration
We use satellite-acquired SAR data from the Sentinel-1 satellites provided
by the European Space Agency (ESA) through the Alaska Satellite Facility SAR
Data Center repository. The InSAR process using ISCE software includes
multilooking, topography removal, flattening, filtering, unwrapping,
georeferencing and re-wrapping steps to obtain a longer-term baseline for
the regional subsidence velocity (Fig. 1).
We examined the effects of both earthquakes on the up to 360 mm yr-1
subsidence process in Mexico City using Sentinel-1 derived subsidence. As a
baseline for pre-seismic deformation, we generated two 24 d SAR
interferograms of data acquired between 19 July–12 August (Fig. 3a) and
12 August–5 September (Fig. 3b). These interferograms, covering the Mexico
City Metropolitan area, show roughly 0.5–1 π phase change (light blue to
purple), which is equivalent to 0.7–1.4 cm surface change in the highly
subsiding regions within the city (Fig. 1). As the observed subsidence
occurred over a period of 24 d, the two interferograms reflect subsidence
rate of roughly 100–200 mm yr-1. We also processed a third differential
interferogram of data acquired on 5 and 29 September, which
encompasses the integrated deformation for both the Mw 8.2 and Mw 7.1
earthquakes (Fig. 3c). This interferogram shows that the previously
mentioned areas underwent an increase subsidence with much larger spatial
coverage in comparison to the previous interferograms (Fig. 3a and b)
which are 24 and 48 d older. The change in subsidence velocities (up to
∼ 33 mm total vertical displacement, equivalent to
∼ 38 % rate increase over a 24 d time window) and areal
extent integrated during both seismic events (Fig. 4) indicates that most
of the seismically triggered subsidence centered on these rapidly subsiding
areas.
It is indeed possible that the energy released during these earthquakes is
responsible for a distinctive deformation velocity pattern that is not
shared by other sectors of the city where the underlying lacustrine sediment
package is either thinner or absent.
Subsidence velocity difference obtained by subtracting the 24 d
subsidence vertical velocity field calculated from Fig. 3b prior to the
Mw 8.2 Tehuantepec and the Mw 7.1 Puebla, September 2017 earthquakes vertical
velocity field calculated from Fig. 3c.
A closer analysis of this circumstance shows that a large portion of the
reported subsidence associated damage correlates with the presence of
preexistent, subsidence-related faults (CENAPRED, 2017). Moreover, we find
evidence of phase discontinuities in the 5–29 September
interferogram, which also correspond to these areas around the lower slopes
of the Sierra de Santa Catarina.
We conclude that the seismic energy from both earthquakes induced a fast
soil consolidation and triggered the coseismic faulting of the preexistent
subsidence related faults. This circumstance not previously observed 32 years ago during the Mw 8.1 19 September 1985 earthquake creates a new
variable that needs to be addressed in future updates to the building codes
and urban zoning considerations in Mexico City.
Conclusions
Our data makes it plausible that the seismic energy released by the Mw 8.2 8 September 2017, and the Mw 7.1 19 September 2017 earthquakes induced
fast soil consolidation within a short time span, and triggered slip on the
preexisting subsidence associated faults. This effect has not been
previously documented but may be triggered again during another strong
earthquake. Future hazard assessments for Mexico City should consider this
triggering mechanism for the assessment and the inclusion shallow faulting
for future urban building code and land use.
Data availability
Sentinel-1a and b SAR data used on this work was provided by the European Space Agency (ESA) through the Alaska Satellite Facility SAR Data Center (ASF SDC) at https://www.asf.alaska.edu/ (last access: 14 April 2020, Alaska Satellite Facility SAR Data Center, 2020).
Author contributions
DSR provided formal analysis and methodology, ECC provided conceptualization
of the project, investigation, resources and writing of original draft and
supervision, EFT provided methodology, investigation and validation, EH
provided interferometric analysis during his doctoral studies in University
of Miami, SW provided investigation, supervision and review and editing of
draft, LST provided investigation and validation.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
Acknowledgements
We thank Yunjun Zhang, Yoangel Torres, Talib Oliver and Leonardo Reyes for their help. Enrique Fernández-Torres acknowledges support from CONACyT and Dario Solano-Rojas acknowledges support from CONACyT and Fulbright-García Robles. SAR data was kindly provided by the European Space Agency (ESA). Part of the InSAR processing was done on UNAM-DGTIC's Miztli supercomputing facility with processing time granted by LANCAD and CONACyT.
Financial support
This research has been supported by the Universidad Nacional Autónoma de México, Dirección General de Asuntos del Personal Académico, DGAPA-PAPIIT projects IV100215 and IN104818-3 and Consejo Nacional de Ciencia y Tecnología (CONACyT) projects 256012 and 005955 to Enrique Cabral-Cano and NASA project NNX12AQ08G to Shimon Wdowinski.
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